YZF overbore (444) kit from Eric Gorr

Well I finally have enough hours on my bike with the Wiseco big bore/high compression kit to give an opinion of its worth. I think I paid $350, all included. It was definitely worth every penny. If this mod doesn’t cause any future reliability problems that is. I have already blown a head gasket but that can probably be attributed to not retorquing the head bolts. I have about 8 hours on the second gasket. So I recommend retorquing the head bolts after the first few hours on a new gasket. :)

At any rate here is my impression of the kit. Keep in mind that I went oversize AND higher compression (13.5:1) at the same time. I have no idea which mattered more but I’d certainly recommend a higher compression piston to anyone replacing their stocker that rides at elevation. My bike has been ridden at 4,000 ft. and above on plain pump gas with not one sign of pinging.

For some perspective I don’t ever race motocross but I will probably race 5 or 6 off road things in a year. Extra punch is always welcome out here in the desert, the trails I ride are deep sand between the cacti type of stuff. I have a PC T4, notable because Mr. Gorr mentioned that the extra displacement works well with large diameter headers. Currently I’m geared 14/47 but I think a 46 or 45 may be in the cards (read on).

The best analogy I can offer is the 400 vs. 426 difference. Anybody who has ridden both these bikes can relate to the big bore kit by simply adding the extra OOMPH the 426 enjoys over the 400 to a stock 426. It is a one gear difference. On two occasions I slowed mid-trail and went back down through the gears to make sure I was in fifth. The motor was responding like I was in fourth. This on a favorite trail I ride often that never saw fifth gear before. You might as well just head for fourth or fifth as soon as you get under way and leave it there. Unbelievable torque down low with a scary mid range to accompany it. Shifting and any use of the clutch is simply not needed. Bring on that 520, I wanna drag race!

There is a very steep, long and sandy hill with a short approach that I use to struggle up on my KX 250, third gear fanning the clutch. When I got a KX 500 that was the first thing I tried - fourth gear, no clutch. My 426 required a tad more work but also would pull fourth gear, no clutch with the stock gearing. I can now catch a full second of air time after blasting up the entire thing in fifth, and I’m down to about half throttle at the top (I’m crazy but not stupid). This with gearing that is two teeth taller than stock. That should give you an idea of the difference.

Now for the downside:

It seems to vibrate a bit more but this isn’t readily apparent. I only recently noticed this during a long ride.

It is definitely easier to stall. It may be time for me to finally bite the bullet and install a flywheel weight.

It doesn’t rev quite as readily. This also struck me when riding my (brand new) 426 after getting off a 400. Part of this may be attributed to the new top end but it is pretty well broken in by now. Mind you it will still find the rev limiter, but it bounces off it like a tennis ball instead of a crazy ball.

It did not make the bike easier to start :D


By the way I would recommend the services of Mr. Gorr based solely on his telephone manners. He almost always answers the phone and is happy to answer any and all questions.


Here’s hisWebsite too.

My '98 400 is coming due for it's first top end, and I'm considering the 444 kit. Not that I need it, but because it's there :-)

Of course, my bike will need a new rod and new 426 cylinder (about $36 and $225), plus the Wiseco piston set and machine work.

The gear ratios are closer in the 426, so I wonder if the gears are *stronger* also. And certainly, I'll need to replace my 3 year old clutch with a premium model.

Actually, I think I'd like to put the WR 1st and 5th gear in my YZ, since I only use 2nd, 3rd and 4th on the motocross track, and need the extreme low gear and high speed for my infrequent trips to Baja.



Thanks for the report. I was wondering how things were going with the big bore kit. I don't want to sound like a sales pitch for Eric Gorr, but he has done all my dirt bike engine machining back since '87 when his shop was 20 min away from my house. Now he is in Madison WI (about 2 1/2 hours away) and I would still not take it any other place. His knowlege and additude comes second to none. A real hard to find combination these days.

just my $.02



I was seriously considering a WR 5th gear swap since I had my cases and tranny apart already. Unfortunately while talking to a mech. over the phone I let him talk me out of it. This particular guy at NCY (Escondido, CA), where I mail order most parts from, didn’t think swapping the two gears in (wheel and pinion) would work. He seemed to think that the entire gear set was sized to fit together and that the two WR gears would not mesh with the rest of the gears.

Now that I think about it that doesn’t make much sense. I’ve heard enough about this that I bet that guy was wrong, at least he was right about some things. But I would still consider a WR 5th, especially with the extra punch I’ve got to pull it, so let us know if you ever do this gear swap. But to swap first gear in I think you will have to buy a new main shaft since first gear is integral to the shaft.


While I have only dealt with him over the phone I am impressed with how helpful Eric Gorr is. You are right, It is pretty refreshing to run across someone so knowledgeable with such a good attitude. I probably get bad advice (see above) delivered with a bad attitude most of the time. I’m not sure why I put up with it…

…but it is all relative. After all, when I asked how to install my new crank, the service dept. mgr. (and owner’s brother) of the Yamahama place in El Paso replied, “Just use a hammer.” :)

You may be right, but I’m guessing the valve area just can’t fill the cylinder adequately at higher rpms.

I don’t think the 400 cylinder will accept a 97 mm bore as Mr. Gorr indicated a 426 cylinder was necessary for the 97 mm Wiseco. I do know that Thumper Racing used to make a 97 mm kit for the 400 but I believe that used a sleeve insert instead of the bore and plate that Eric Gorr does.

Also, I’m not sure what it would cost but to change the rod (w/ bearing about $70) the crank needs to be rebuilt. I purchased a new crank assembly (rod included) for $300 from NCY. My old crank is bent but I’m sure its rebuildable. I may run across a clapped-out 400 some day and swap my (retrued) crank, (not overly worn) piston and a new 426 barrel ($240) for an instant 426 conversion. Supposing this ever happens Taffy do you (or anybody else) have any idea who can true my crank?

Before you say Falicon I was told they no longer rebuild singles.

On the 1st and 5th gear swapping, I read posts a long time ago from guys who changed 1,2 and 5 in their YZ's to get WR ratios. It seemed that it worked and they were happy with the results.

Hick, how about the WB oversize intake valves?

can you use the cylinder from a 400 tto get to 444-surely you can? i know that the rod needs changing.

i wouldn't pit my knowledge against mr gorr but i would think the reason that the bikes don't rev so quick is because the squish MUST be modified to a two-teir squish. if it isn't done the engine can't roll over.


Originally posted by Scott F:

Hick, how about the WB oversize intake valves?

Yeah, I’m considering that. More expensive than the big bore kit though, WB’s motor services seem pretty expensive. I wonder how much this would help the overbore?

I’m actually so satisfied that I doubt any more motor mods are in my future. Believe it or not but I think I’m done spending money on my bike (until the next thing breaks that is).

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